7. PROJECT SUMMARY Over 15 million Americans now have severe obesity, i.e. a body-mass index (BMI) ?35 kg/m2. Patients with severe obesity have 5-times higher prevalence of type 2 diabetes, and twice the annual medical costs of those with normal body weights. Bariatric surgery is the most effective treatment for severe obesity, but is invasive, costly, and associated with short and longer-term complications. The vertical sleeve gastrectomy (VSG) is a newer bariatric procedure that has substantially increased in use recently, now accounting for approximately half of procedures worldwide. However, very little is known about longer-term outcomes after VSG, including the durability of its impact on diabetes, and its potential for reducing cardiovascular morbidity and mortality. Of particular importance for patients considering surgery, the risks and benefits of VSG compared to the ?gold standard? gastric bypass (RYGB) procedure are not well understood. The proposed study will use a nationwide commercial insurance claims dataset to evaluate clinical and health care use outcomes among over 10,000 patients undergoing VSG, compared to 10,000 patients undergoing RYGB, between 2005 and 2017, with up to 8 years of post-operative follow-up. Rigorous observational study designs with propensity-score matching will be used to compare the effect of VSG and RYGB on: clinical outcomes, including diabetes remission and relapse, myocardial infarction, stroke and death; health care use outcomes including emergency department visits, hospital days, and reoperation or revisional surgery; and health care costs including standardized total, prescription and out-of-pocket costs. Furthermore, to assist with procedure selection, exploratory analyses will investigate the optimal surgical type for patients with certain key characteristics (e.g. by age group, diabetes severity). To minimize risk of confounding by indication, cutting-edge propensity-score matching algorithms will be employed and rigorous longitudinal designs such as interrupted time series and survival analysis will be constructed. This will be the largest claims-based comparative study of these two procedures, with potential to significantly impact clinical practice in bariatric surgery. Findings could inform more evidence-based practice and facilitate shared clinical decision-making by improving the ability of patients and surgeons to select a surgical approach that is optimized to patients? preferred health outcomes and personal characteristics.